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A balanced amplifier created for use in 900-MHz cell towers can contribute to simpler cellular system design.

Ampilifer Assembly

Since noise figure is critical in this application, wirewound chip inductors were used instead of multilayer inductors. Beside bias insertion, the aforementioned inductors and series capacitors, C3 and C16, also form high-pass networks to roll off unneeded low frequency gain. The long bondwires, Lw, which connect the drain to the package leads also serve as prematching structures.

The shutdown function block consists of transistor switch in series with the active bias. Shutdown is initiated by applying a high logic (≥ 2 V) at Vsd1/2 to open the switch. Conversely, a low logic ( i.e., Vsd1/2 ≤ 500 mV), turns on the amplifiers. Transitioning from normal to shutdown mode takes less than 50 ns if the large (≥ 0.1 μF) decoupling capacitors C6, C8, C20, C22, C23, and C24 are omitted. However, these capacitors are generally recommended because they aid low-frequency stability and dampen supply transients.

Although a Lange coupler is the most compact type of coupler component, its extremely fine trace width is not suitable for PCB implementation. As a result, the balanced LNA design uses commercially available multilayer couplers, X1-2, to perform signal combining and splitting. A larger coupler measuring 6.4 x 5.1 mm is used at the input because of its lower loss, while a smaller coupler measuring 2.0 x 1.3 mm is used at the output to save space and cost. To ensure that the input return-loss requirement of 21 dB or better can be met without fail in volume production, the critical parameters were identified through a Monte Carlo analysis. Subsequently, two controls were instituted: correlating the amplifiers’ input match to |S11a - S11b| < 0.025 and ensuring that the input coupler’s isolation is greater than 23 dB. The first control was satisfied by using adjacent chips, while the second required specifying a high-isolation input coupler.10 The output coupler is not critical. As a result, the worst-case input return loss measured in 500 samples was 21.5 dB (Fig. 2).

The PCB for the balanced LNA is built on RO4350 circuit-board material (Fig. 3) from Rogers Corp. Wherever practical, microstrip linewidths were dimensioned for a characteristic impedance of 50 Ω. However, circuit traces next to the input coupler’s mounting pads are necked down following the manufacturer’s recommendations to compensate for the pads’ parasitic capacitances. An FR-4 circuit-board layer was added to the 10-mil-thick PCB to increase the stack height to 1.6 mm. The input and output microstrip lines are transitioned to coaxial connectors using edge-launched SMA receptacles. All performance results are referenced to the coaxial ends.